The use of chelated radionuclide (Samarium-153-EDTMP) to modulate phenotype of tumor cells and enhance T-cell-mediated killing. Exposing human tumor cells to sublethal doses of external beam radiation upregulates expression of tumor antigen and accessory molecules, rendering tumor cells more susceptible to killing by antigen-specific cytotoxic T lymphocytes (CTLs). This project explored the possibility that exposure to palliative doses of a radiopharmaceutical agent could alter the phenotype of tumor cells to render them more susceptible to T-ell-mediated killing. Here, 10 human tumor cell lines (4 prostate, 2 breast, and 4 lung) were exposed to increasing doses of the radiopharmaceutical samarium-153-ethylenediaminetetramethylenephosphonate (153Sm-EDTMP) used in cancer patients to treat pain due to bone metastasis. Fluorescence-activated cell sorting analysis and quantitative real-time polymerase chain reaction (PCR) analysis for expression of five surface molecules and several tumor-associated antigens involved in prostate cancer were done. LNCaP human prostate cancer cells were exposed to153Sm-EDTMPand incubated with tumor-associated antigen-specific CTL in a CTL killing assay to determine whether exposure to 153Sm-EDTMP rendered LNCaP cells more susceptible to T-cell-mediated killing. Tumor cells up-regulated the surface molecules Fas (100% of cell lines upregulated Fas), carcinoembryonic antigen (90%), mucin-1 (60%), major histocompatibility complex (MHC) class I (50%), and intercellular adhesion molecule-1 (40%) in response to 153Sm-EDTMP. Quantitative real-time PCR analysis revealed additional upregulated tumor antigens. Exposure to 153Sm-EDTMP rendered LNCaP cells more susceptible to killing by CTLs specific for prostate-specific antigen, carcinoembryonic antigen (CEA), and mucin-1. Doses of 153Sm-EDTMP equivalent to palliative doses delivered to bone alter the phenotype of tumor cells, suggesting that 153Sm-EDTMP may work synergistically with immunotherapy to increase the susceptibility of tumor cells to CTL killing. The use of radiolabeled monoclonal antibody to enhance vaccine-mediated T-cell responses. Radiolabeled monoclonal antibodies (mAb) have demonstrated measurable antitumor effects in hematologic malignancies. This outcome has been more difficult to achieve for solid tumors due, for the most part, to difficulties in delivering sufficient quantities of mAb to the tumor mass. Previous studies have shown that nonlytic levels of external beam radiation can render tumor cells more susceptible to T cell-mediated killing. The goal of these studies was to determine if the selective delivery of a radiolabeled mAb to tumors would modulate tumor cell phenotype so as to enhance vaccine-mediated T-cell killing. Here, mice transgenic for human CEA were transplanted with a CEA expressing murine carcinoma cell line. Radioimmunotherapy consisted of yttrium-90 (Y-90)-labeled anti-CEA mAb, used either alone or in combination with vaccine therapy. A single dose of Y-90-labeled anti-CEA mAb, in combination with vaccine therapy, resulted in a statistically significant increase in survival in tumor-bearing mice over vaccine or mAb alone;this was shown to be mediated by engagement of the Fas/Fas ligand pathway. Mice receiving the combination therapy also showed a significant increase in the percentage of viable tumor-infiltrating CEA-specific CD8+ T cells compared to vaccine alone. Mice cured of tumors demonstrated an antigen cascade resulting in CD4+ and CD8+ T-cell responses not only for CEA, but for p53 and gp70. These findings show that systemic radiotherapy in the form of radiolabeled mAb, in combination with vaccine, promotes effective antitumor response, which may have implications in the design of future clinical trials.